JERUSALEM, Israel -- For the second time in as many days, a mortar shell fired from Syria exploded in the Golan Heights. Israel returned fire, possibly targeting rebel forces with an anti-tank missile.

On Sunday, Israel Defense Forces spokesperson Lt. Col. Avital Leibovich said the shelling was spillover from Syria's civil war, but the IDF fired back as a warning. Despite that, another mortar hit the Golan on Monday and again Israel returned fire.

Meanwhile, the Golan Regional Council instructed residents to get ready for any eventuality.

The council represents 33 mostly agricultural communities on the Golan Heights, including 19 moshavim and 10 kibbutzim.

Residents of Moshav Alonei HaBashan set about preparing bomb shelters and making sure emergency equipment is in good repair and ready to use, YNet reported.

"Everyone who lives near the border [with Syria] should be prepared," said Yiska Dekel, head of the Alonei HaBashan committee, adding that no one is "hysteric" yet. "The shelter wasn't even open until today, but now we're more alert."

Defense Minister Ehud Barak said troops in the north have been told to return fire if fired upon. When a mortar shell exploded in the Golan's eastern sector Sunday, the IDF responded by firing a Tammuz missile at a Syrian post. It was the first time since 1973 that Israel fired on Syria.

Sunday's mortar shell attack was the latest of several such incidents, but Barak didn't define it as a "cardinal threat." Instead he called it a "potential threat in an unpredictable area."

At least one local resident recognized the potential for escalation between Syria and Israel, saying Sunday's incident might "evolve into a series of continues strikes between the two sides."

Meanwhile in neighboring Lebanon, three people were killed in gunfights between Sunni and Shiite Muslims Sunday. Fighting erupted in the port city of Sidon when Shiites displayed banners marking the Ashura holiday, a 10-day mourning period marking the death of the prophet Mohammed's grandson.

Lebanese Sunni Muslims back Syria's rebel forces while Shiites back President Bashar Assad, whose Alawite sect is an offshoot of the Shiite sect.